In general, a composition of the type which comprises an alkali-soluble resin and a naphthoquinonediazide compound as photosensitive material has been used as positive photoresist composition. For instance, the combinations of novolak-type phenol resins with naphthoquinonediazido-substituted compounds are disclosed as compositions of the aforesaid type, e.g., in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470, and the combinations of novolak resins prepared from cresols and formaldehyde with trihydroxybenzophenone-1,2-naphthoquinonediazide sulfonic acid esters are described as the most typical compositions in L. F. Thompson, Introduction to Microlithography, No. 2, 19, pp. 112-121, ACS publisher.
The positive photoresist as described above is constituted basically of a novolak resin and a quinonediazide compound. Therein, the novolak resin acts so as to provide high resistance to plasma etching and the naphthoquinonediazide compound functions as a dissolution inhibitor. Further, the naphthoquinonediazide has a characteristic of generating a carboxylic acid upon irradiation with light to lose the ability to inhibit dissolution, thereby heightening the solubility of a novolak resin in alkali.
From the aforementioned points of view, a large number of positive photoresist compositions which each comprise a novolak resin and a naphthoquinonediazide type photosensitive substance have hitherto been developed and put to practical use. In the microlithography for the lines having a width of the order of 0.8-2 .mu.m, those compositions have accomplished satisfactory results.
On the other hand, the integration degree of an integrated circuit has become higher and higher in recent years, so that the microlithography for superfine patterns having a line width of half micron or below has come to be required in the production of semiconductor substrates for VLSI and the like. In order to achieve the resolution required therein, the wavelengths used in an exposure apparatus for photolithography were getting shorter, and nowadays the utilization of far ultraviolet light and excimer laser beams (XeCl, KrF, ArF) is being examined.
In a case where a conventional resist comprising a novolak resin and a naphthoquinonediazide compound is used for forming patterns by the lithography using far ultraviolet light or an excimer laser beam, it is difficult for the light to reach the lower part of the resist because a novolak resin and naphthoquinonediazide have strong absorption in the far ultraviolet region, so that such a resist has low sensitivity and can merely provide a pattern profile having a tapered shape.
As one means for solving the aforementioned problems, the chemically amplified resist compositions disclosed, e.g., in U.S. Pat. No. 4,491,628 and European Patent 0,249,139 can be used. The chemically amplified positive resist compositions refer to the pattern forming materials of the type which generate acids in the irradiated area upon exposure to radiation, such as far ultraviolet light, to cause a reaction utilizing these acids as catalyst, thereby making a difference of solubility in a developer between the areas irradiated and unirradiated with active radiation. By virtue of this solubility difference, a pattern can be formed on a substrate coated with a material of the foregoing type.
As examples of a chemically amplified resist composition, mention may be made of the combination of a compound capable of generating an acid by photolysis (which is called a photoacid generator, hereinafter) with acetal or an O, N-acetal compound (JP-A-48-89003, wherein the term "JP-A" means an unexamined published Japanese patent application"), the combination of a photoacid generator with an orthoester or amidoacetal compound (JP-A-51-120714), the combination of a photoacid generator with a polymer containing acetal or ketal groups in its main chain (JP-A-53-133429), the combination of a photoacid generator with an enol ether compound (JP-A-55-12995), the combination of a photoacid generator with an N-acyliminocarbonic acid compound (JP-A-55-126236), the combination of a photoacid generator with a polymer containing orthoester groups in its main chain (JP-A-56-17345), the combination of a photoacid generator with a tertiary alkyl ester compound (JP-A-60-3625), the combination of a photoacid generator with a silyl ester compound (JP-A-60-10247) and the combinations with a photoacid generator with silyl ether compounds (JP-A-60-37549 and JP-A-60-121446). Those combinations have a quantum yield greater than 1 in principle, so that they exhibit high sensitivity.
As examples of a similar system to the above, which is stable upon storage at room temperature but decomposes by heating in the presence of an acid to acquire solubility in an alkali, mention may be made of the systems described, e.g., in JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642, Polym. Eng. Sce., vol. 23, p. 1012 (1983); ACS. Sym., vol. 242, p. 11 (1984); Semiconductor World, the November number, p. 91 (1987); Macromolecules, vol. 21, p. 1475 (1988); and SPIE, vol. 920, p. 42 (1988); wherein compounds capable of generating acids by exposure to light are combined with tertiary or secondary alkyl (e.g., t-butyl, 2-cyclohexenyl) esters or carbonate compounds. Such systems also have high sensitivity, and their absorption in Deep-UV region is weak, compared with a naphthoquinonediazide/novolak resin system. Thus, the use of those systems can also be effective in shortening the wavelengths of a light source.
The foregoing chemically amplified positive resist compositions can be grouped into two main classes, namely a class of three-component systems comprising an alkali-soluble resin, a compound capable of generating an acid by exposure to radiation (a photoacid generator) and a compound which contains acid-decomposable group(s) and can inhibit the dissolution of an alkali-soluble resin (a dissolution inhibitor) and a class of two-component systems comprising a resin containing groups which are decomposed by the reaction with an acid to become soluble in an alkali and a photoacid generator.
Such a two- or three-component positive resist of chemical amplification type forms a resist pattern by undergoing development after a heat treatment in the presence of an acid which is generated from a photoacid generator by exposure to light.
With respect to photoacid generators used in the aforementioned positive resist of chemical amplification type, although an N-oxime sulfonate, an o-nitrobenzyl sulfonate, a pyrogallol trismethanesulfonate and so on were known, the sulfonium or iodonium salts of perfluoro Lewis acids, such as PF.sub.6.sup.-, AsF.sub.6.sup.- and SbF.sub.6.sup.-, as described in JP-A-59-45439 and Polym. Eng. Sci., 23, 1012 (1983) have been used as typical photoacid generators since they have high photolysis efficiency and excellent image forming capability.
In cases where such photoacid generators are used in resist materials for semiconductors, however, contamination with phosphorus, arsenic and antimony arising from the counter anions of those photoacid generators becomes a problem.
As sulfonium and iodonium compounds free from such contamination, the salts whose counter anion is trifluoromethanesulfonic acid anion, such as those described in JP-A-63-27829, JP-A-2-25850, JP-A-2-150848, JP-A-5-134414 and JP-A-5-232705, are used.
However, the use of those salts gives rise to a problem such that the resist pattern is thinned and the profile thereof comes to have a T-shape top with the passage of time in the period from exposure to heat treatment.
As a means for solving the above problem, the photosensitive composition using an acetal group-protected hydroxystyrene resin and a particular acid generator is described in JP-A-5-242682. This means is effective in controlling the formation of T-shape top but insufficient for preventing the resist pattern from being thinned with the lapse of time in the period from exposure to heat treatment.
Further, the resist composition using an acid-labile compound containing a ketal component and a photoacid generator is described in JP-A-7-140666. This composition also has an effect on prevention of the T-shape top formation, but it does not produce sufficient effect in controlling the thinning of resist pattern. In addition, that composition has a serious drawback in storage stability.